The present invention relates of a delay circuit using charging/discharging of a capacitor and to a device using the delay circuit.
An example of a conventional delay circuit using charging/discharging of a capacitor is shown in FIG. 2. First, the operation of this delay circuit will be described.
At an initial state, since a switch 1 is turned ON (closed) and a switch 2 is turned OFF (opened), a terminal A to which a capacitor 7 is connected has a potential of (V1xe2x88x92V2). In order to start the operation of this delay circuit, a signal is supplied to an input terminal 11. When the signal enters the input terminal 11, a control circuit 8 turns OFF the switch 1 and turns ON the switch 2. A constant current source 12 is connected to the terminal A, and the electric charge of the capacitor 7 also connected to the terminal A is discharged.
When the potential of the terminal A reaches a potential V3 of a reference voltage 5, the output of a comparator 9 is inverted. Since it takes some time to discharge, this circuit is used as a delay circuit. Here, a time from the start of the operation of the control circuit 8 to the inversion of the output of the comparator 9 becomes a delay time.
This operation is shown in FIG. 3. The horizontal axis indicates time, and the vertical axis indicates voltage. At the initial state, although the potential at the terminal A in FIG. 2 is (V1xe2x88x92V2), the capacitance is discharged through constant current from a time ta when the signal is supplied to the input terminal 11, so that the potential at the terminal A is lowered. Thereafter, when the potential at the terminal A reaches the potential V3 of the reference voltage 5, the output voltage of the comparator is inverted.
When a capacitance value of the capacitor 7 is C0, and a constant current value of the constant current source 12 is I0, a delay time T0 may be represented by the following Equation 1:
T0=C0xc3x97(V1xe2x88x92V2xe2x88x92V3)xc3x97I0xe2x80x83xe2x80x83[Equation 1]
When the capacitance value C0 is made large or the constant current value I0 is made small, the delay time can be extended. On the other hand, when the capacitance value C0 is made small or the constant current value I0 is made large, the delay time can be shortened.
In the case of the delay circuit using charging/discharging of the capacitor shown in FIG. 2 as the prior art, there are problems as described below.
In the delay circuit described thus far, a problem occurs when the terminal A to which the capacitor 7 is connected is short-circuited to an abnormal voltage such as the potential V1. In the case where the terminal A to be connected to the capacitor 7 comes to have a voltage higher than (V1xe2x88x92V2), the inverted input terminal of the comparator 9 does not become lower than V3. Thus, the output of the comparator 9 is not inverted. This means that the delay is not realized, and the delay time is extended without limit. FIG. 4 shows potentials at this time. The terminal A to which the capacitor 7 is connected, is connected in an abnormal state and has the potential V1. Thus, the voltage at the terminal A is always at the potential V1, and the output of the comparator 9 is not inverted.
This delay circuit is also used as a protecting circuit or an erroneous operation preventing circuit. For example, in a lithium ion battery, when a voltage becomes too high, there is a risk that it may cause a fire. Thus, an overcharge preventing circuit is provided. The delay circuit can also be used for the overcharge preventing circuit. In addition a delay circuit is also used in a charger for lithium ion batteries to set a charging time (charge stop circuit) and the like. Thus, when the terminal A to which the capacitor 7 is connected as shown in FIG. 1 is short-circuited to a voltage higher than (V1xe2x88x92V2), the overcharge preventing circuit or the charge stop circuit does not work, and a very dangerous state occurs. If the delay of the charger does not work, charging is continues permanently, and heat generation of the charger and the lithium ion battery is advanced. This causes a very dangerous state such that a fire or heat generation of the apparatus may occur.
In a circuit provided for protection or improvement of safety, even if a problem occurs, some protection must work. A so-called fail-safe must be realized. In the case of the present invention, it is required that even if the delay time is shortened, an output voltage is inverted and a signal is transmitted to the outside.
In a delay circuit using charging/discharging of a capacitor to set a delay time, there is added a voltage detecting circuit for detecting whether a terminal voltage of the capacitor has exceeded a predetermined voltage range.